This invention relates to traction control systems for motor vehicles for automatically braking the vehicle wheels so as to prevent wheelspin. More specifically, the invention pertains to a wheel slip responsive traction control system offering a choice between two or more different drive modes. The traction control system of the invention finds a typical application to a four wheel drive vehicle, making it possible to drive the vehicle in either a "normal drive" mode, a "front drive" mode or a "rear drive" mode.
Vehicle traction control in response to the rate of wheel slip is per se not new in the art. Generally, in this type of traction control system, vehicle traction is so controlled as to limit the slip of the drive wheels at a rate (S1 in FIG. 3 of the drawings attached hereto) where the driving force and lateral force of the vehicle are equally high. Wheelspin is thus prevented by assuring the high lateral force without substantially sacrificing the driving force at driving on cornering.
Traction control by the above discussed conventional method enhances vehicle stability but diminishes maneuverability as a sporty like, for the lateral force greatly affects steerability. Take a four wheel drive vehicle for example. The vehicle will gain in maneuverability, behaving like a rear drive vehicle, if the lateral force of the rear pair of drive wheels is lessened. If the lateral force of the front pair of drive wheels is decreased, on the other hand, then the vehicle will gain in stability and behave like a front drive vehicle. As far as the applicant is aware, there has been suggested no system that realizes selective traction control in either of such different drive modes.
U.S. Pat. No. 3,893,535 and Japanese Patent Laid-Open Publication No. 62-87630 represent examples of prior art traction control systems. The cited prior art systems are similar in that, upon detection of wheelspin, the engine output torque is automatically decreased to overcome the wheelspin. Traction control through changes in engine output torque is objectionable because it is incapable of independent control of the front and rear pairs of wheels. The provision of two or more different drive modes necessitates the independent traction control of the two pairs of wheels.